The present invention relates to a liquid crystal drive circuit for driving a liquid crystal display element having scanning and signal electrodes arranged in matrix form.
Liquid crystal television receivers having liquid crystal display panels as display screens have been commercially available as portable compact television receivers in recent years. Liquid crystal color television receivers with color liquid crystal panels are being developed. Various methods can be utilized for color liquid crystal display. As shown in FIG. 1, primary color filters 1 of R (red), G (green), and B (blue) are connected to signal electrodes to constitute a typical conventional color liquid crystal panel 2. Color display is performed by a combination of three primary colors. Referring to FIG. 1, reference numeral 3 denotes a scanning electrode driver; 4, an R signal electrode driver; 5, a G signal electrode driver; 6, a B signal electrode driver; and 7, a liquid crystal voltage generator for supplying operating voltages V0 to V5 to each of drivers 3, 4, 5, and 6.
FIG. 2 is a sectional view showing part of color liquid crystal panel 2. A pair of glass plates 11a and 11b are spaced apart by a predetermined distance, and liquid crystal 12 is filled therebetween to constitute panel 2. Transparent scanning electrode 13 is formed on the inner surface of glass plate 11a, and signal electrodes 14, 15, and 16 of R, G, and B signals are arranged on glass plate 11b. R, G, and B color filters 17, 18 and 19 are formed on signal electrodes 14, 15, and 16, respectively. Deflecting plates 20a and 20b are formed on outer surfaces of glass plates 11a and 11b, respectively.
When color filters 17, 18, and 19 are formed on signal electrodes 14, 15, and 16, these filters are formed in units of colors. It is very difficult to uniformly form three different filters, causing variations in filter thickness. The variations cause differences in effective values of voltages at the liquid crystal even if a single voltage is applied thereto, thus degrading display quality. Japanese Utility Model Disclosure (Kokai) No. 61-124 describes a color liquid crystal panel drive circuit for providing a high quality display with good reproducibility even if the thicknesses of the R, G, and B filters are not uniform.
In the color liquid crystal panel drive circuit in
Japanese Utility Model Disclosure (Kokai) No. 61-124, at least two of R, G, and B bias voltage generators have voltage regulators to variably regulate bias voltages, respectively.
Referring to FIG. 3, reference numeral 30 denotes a color liquid crystal panel. Panel 30 is driven by R signal electrode driver 31, G signal electrode driver 32, B signal electrode driver 33, and scanning electrode driver 34. Drivers 31, 32, 33, and 34 receive bias voltages from R, G and B bias voltage generators 35, 36, and 37. Assume a 1/13 bias voltage. As a generator 35 comprises five resistors series-connected between ground line GND and power source voltage Vcc, a potential difference between GND and Vcc is divided into 13 portions under the conditions where the resistance of the central resistor is 9R, and the resistances of other resistors are R each, thereby obtaining voltages V0 to V5. Voltages V0, V1, V4, and V5 are supplied to scanning electrode driver 34, and voltages V0, V2, V3, and V5 are supplied to driver 31. A bias ratio is determined by a duty ratio of color liquid crystal panel 30. If the duty ratio is 1/N, bias ratio b is defined as b=.sqroot.N+1. Generator 36 comprises three series-connected resistors and has one end connected to ground line GND and the other end connected to the sliding terminal of variable resistor 361. Resistor 361 is connected between power source voltages Vcc and Vcc'. Its regulated voltage is supplied to generator 36. Generator 36 generates voltages V2', V3', and V5' under the conditions where the resistance of the central resistor is 9R and the resistances of other resistors are R. Voltages V2', V3', and V5' as well as voltage V0 are supplied to G signal electrode driver 32. B bias voltage generator 37 comprises three series-connected resistors and has one end connected to ground line GND and the other end connected to the sliding terminal of variable resistor 371. Resistor 371 is connected between power source voltages Vcc and Vcc", and its regulated voltage is supplied to generator 37. Generator 36 generates voltages V2", V3", and V5" under the conditions where the resistance of the central resistor is 9R and the resistances of other resistors are R. Voltages V2", V3", and V5", as well as voltage V0, are supplied to B signal electrode driver 33.
Display data system signal lines are omitted from FIG. 3.
In the conventional example in FIG. 3, voltage regulators of variable resistors 361 and 371 are respectively arranged in G and B bias voltage generators 36 and 37, respectively, thereby controlling the display colors on color liquid crystal panel 30. If resistor 361 is variably operated, a voltage supplied to generator 36 varies so that voltages V2', V3', and V5' supplied to G signal electrode driver 32 vary. If resistor 371 is variably operated, a voltage supplied to generator 37 varies so that voltages V2", V3", and V5" supplied to B electrode driver 33 vary. Therefore, even if variations in thicknesses of R, G, and B filters occur, the effective values at the R, G, and B liquid crystal components can be set equal to each other, thereby obtaining high display quality.
However, when variable resistor 361 is adjusted to change R, G, and B signal electrode voltages, the effective voltage (X - Y) applied to the liquid crystal element during nonselection of the scanning electrode varies. More specifically, as shown in FIG. 4, if nonselection high level voltage V1 of the scanning electrode is defined as a reference, a biasing component of selection high level voltage V0 of the signal electrode differs from that of nonselection high level voltage V2 thereof. If nonselection low level voltage V4 of the scanning electrode is given as a reference, a biasing component of selection low level voltage V5 of the signal electrode differs from that of nonselection low level voltage V3 thereof. The liquid crystal cannot be driven according to a voltage averaging method using, e.g., 1/13 biasing. For this reason, if a color liquid crystal display is constituted by a matrix type liquid crystal display element, image degradation such as tailing occurs. U.S. Pat. No. 4,518,654 describes a liquid crystal television and illustrates a liquid crystal drive waveform. In addition, U.S. Pat. Nos. 3,945,000, 3,900,742, 3,936,676, 3,896,430, and 4,038,564 describe techniques for generating multi-level voltage signals for driving liquid crystals.
Further, Japanese Patent Publication No. 61-20000 discloses the technique of dividing a resistor to provide output signals of different voltages.